Correctly notify addons when a player has died

[BytewaveMLP]

Currently, GMAU provides no way to notify addons if a player has died/been reported dead. This means that addons such as PointShop for example have no way of knowing when to destroy trails for "dead" players, as it relies on the PlayerDeath hook to know when to destroy the trail. There are a couple potential solutions here:

• Call PlayerDeath, and provide an override for Player:IsAlive. This is the most standard way to handle a player's death, but it would also mean invoking default death behavior from the game as well, which we would need to suppress; we don't want to notify other players of a dead player before their body is found or anything like that.
• Add our own hook, GMAU PlayerDead (SH), which fires on the server when a player is killed, and on the client when a body is marked dead (optional, maybe this should be a separate hook). This means addons need to explicitly listen for this nonstandard hook, but allows us more control over default behavior.